The aims of this study were: i) to identify
differences on Explosive strength (Es), Reactive strength and Explosive Strength
Endurance between two groups of 400m runners with distinct performance levels
and ii) to identify associations between each strength component and the 400m
performance. Fifteen volunteer 400m and 400m hurdles specialists were assigned
to two groups according to their performance level in the 400m (sub 49s and
49s+) and evaluated with the following tests: counter movement jump (CMJ),
fifteen reactivity jumps (15React) and 30 second countermovement jump (30"CMJ).
Significant differences (p <0.01) between the groups for the values achieved
in the CMJ, 30"CMJ and h15-30 (the second half of the 30"CMJ test). The
differences in the 15React were not significant. When data from the two groups
were pooled together significant associations between 400m performance and a
number variables were found. The authors conclude that athletes with different
400m performances tend to present different Es and Es endurance levels and that
significant associations may be found between the level of these strength
components and the competitive performance. Although reactive strength may be
considered important for success in the 400m, they could not find significant
differences in this variable between runners with different
performances.

Introduction

The 400 metres is usually known as a speed endurance event that
demands a capacity to maintain close to maximum speed in an effort of = 45
seconds duration. Vittori (1991) identifies the fundamental functional
requisites for success in this event as a combination of strength, speed and
endurance plus an appropriate effort distribution. As suggested by Harre and
Leopold (1987), in speed endurance and short middle distance events, strength
endurance is the speed strength support for these cyclic movements and therefore
is a determinant of the competitive performance.

Nummela et al. (1992) have studied the effects of fatigue on sprinters performing a 400m
race. On different days subjects ran 400m, 100m, 200m and 300m respectively as
if they where running the event (i.e. with the same pace strategy). To evaluate
the strength level decrease, the subjects performed a drop jump test (from 39cm
height) after the end of each of the referred distances. Comparing rest values
with the those after the runs it was possible to verify the deteriorating force
production over the event. After the 300m there was a 16% drop and after the
400m a 25% decreased was observed. Moreover, the authors found a linear negative
relationship between jump height and lactate levels above
6mmol/1. These results and the results from Nummela et al.
(1994) suggest that during the first 200m of the race muscle properties are
modified (reflected by an increase of contact time and a decrease of explosive
strength in the height of jump) even without a decrease in the running speed.
However from the 200-300m onwards there is a strength reduction, as capillary
lactates concentration attains more than 6mmol/l, and the pH drops.
Additionally, it is possible to observe an increase of the electromyographic
record (EMG) of the muscles that participate in the sprinting action. This
increase in EMG activity during the 400m is caused by the increase in the firing
rate and/or the recruitment of additional motor units to compensate for failure
in contractile capacity of muscle fibres that have already been recruited
(Nummela et al. 1994). In the same study, the authors have found that during the
last phase of the event, ground reaction forces decreased remarkably during the
braking phase, which implies that the musculo-tendon system reduced capacity to
tolerate impact forces. The differences found by the authors between a
sub-maximal 20m run at competition pace and the end of the 400m run suggests
possible changes in stiffness regulation. This possible failure in stiffness
characteristics may reduce the storage of elastic energy during the braking
phase. Indeed, once the strength capacity progressively decreases during the
event (Nummela et al. 1992 and 1994; Hirvonen et al. 1992). due to abolic
changes (Arcelli, 1995; Lacour et al. 1990) or to FT fibres failure (Nummelo et
al. 1994), strength endurance (SE) will prevail as the determinant of success in
this event. The greater the ability of the athlete to oppose fatigue (by
maintaining strength levels), the smaller the drop in speed and consequently the
better the performance (Harre 8 Leopold, 1987). Therefore, SE seems to be a
determinant factor in 400m performance and athletes with the best performances
in the event should also be able to express more power in sprint specific
strength components, i.e. explosive strength (Es), reactive strength (Rs) and
especially Es endurance or speed strength endurance (SSE).
Until the present time, several studies with distinct populations have been
performed in order to investigate the strength components and their importance
in sprint events (Nummela et all, 1992; Delecluse, 1997; Locatelli, 1996; Manou
et all, 2000; Chelly 8 Denis, 2001; Henessy & Kilty, 2001; Miguel, 2003).
However, we have found no data on differences of the strength levels in 400m
runners from distinct performance levels. The main aims of
the present study were to identify differences in Es, Rs and SSE, between two
groups of 400m runners with distinct performance levels and to identify
associations between each strength component and 400m performance (t400) in
speed endurance athletes.

Methods

Subjects Fifteen elite male runners,
400m and 400m hurdle specialists, volunteered to participate in the present
study, after being informed about the aims, risks and advantages of their
participation. The athletes were assigned in two groups according to their
performance level. One group (up49) included the athletes with a 400m season
best (t400) above 49s (n=9; t400=49.86 ±0.43s; age=19.44 ±1.124yr) and the other
group (sub49) included the athletes with a t400m below 49s (n=6; t400=47.66
±0.60s; age=24.17 ±3.48yr).

Procedures All subjects were evaluated
at the end of the Preparatory Phase for three consecutive seasons. Before the
experiments, the subjects were familiarised with the testing procedures. The
subjects were instructed not to engage in high-intensity training during the 48
hours prior to the testing session. The testing session included three tests to
evaluate Es, Rs and SSE according to Bosco (1994) in the following sequence:
three attempts of a counter movement jump (CMJ), with a two minutes rest between
attempts, where the best performance was recorded; after a three minutes rest, a
stiffness test that included 15 reactivity jumps (15React); after a four to five
minutes rest a 30 seconds continuous CMJ (30CMJ). An ergojump system (Bosco
System, Globus, Italy) and the respective software (Ergo Tester) were used. All
the tests were performed in identical conditions, i.e. in indoor rooms with a
range of temperatures between 18 and 25°C and at the same time of day. The
reliability of the tests and instrument was calculated following the author's
indications (Bosco, 1994) and from Ayestaran and Calbet (2001). In the CMJ, the
coefficient of variation true repetition of the test was 0.11 % (22 subjects)
and in the 15React it was 0.16% (10 subjects). For continuous jumps reliability,
we used two mat platforms, one above the other, in 273 jumps performed by 4
subjects. Then we correlated flight times and contact times, which were
respectively r=0.950 (p<0.001) and r=0.997
(p<0.001).

Calculations Based on the several
parameters assessed during the above mentioned tests, the following variables
were considered: height of the rise of the centre of mass (cm) obtained in the
CMJ test (hCMJ), maximum and average values of the estimated power during the
15React (Pmax Rs and Pmed Rs) and average values of heights of the rise of cm of
total and partial parts in the 30CMJ test, i.e. total and each half (h30CMJ, h
0.15 and h 15.30). The calculation of the height of the
rise of the cm (h) was estimated from the flight time of each jump, according to
the Asmussen and Bonde-Petersen formula (Bosco, 1994):

h=
tv_x1.226 (1)

in
which tv is the flight time.

In the 15React, the Mechanical Power of each jump
was calculated using the Bosco-Vittori formula (Gallozi, 1996):

w=
19.86 x tv (2)Tcin which Tc is the contact time.

Pmax Rs was achieved from the average result of the three best
jumps, just as Locatelli (1996) did, and the Pmed Rs from the average of all
jumps.

Data
Analysis Data was analysed with SPSS 11.0 software (SPSS
Science, Chicago, USA) and the graphics were designed with Sigma-Plot 8.0
software (SPSS Science, Chicago, USA). Simple and multiple linear regressions
were used on all appropriate data. The normality of the distribution of the
variables was checked with the Kolmogorov-Smirnov Test. Mean differences were
tested by an Independent Samples T-test. The associations between variables were
determined by the Pearson Product Moment Correlation Coefficient. The
statistical significance was set to p≤0.05. The results are presented as means
± standard
deviations (SO).

Results The mean values of the two groups
are presented in Table 1. The sub49 group obtained the best averages in all
variables of the Es and SSE but not in the Rs. Table 2 presents mean differences
between the two groups studied. Taking into account the
results from all the subjects, associations between t400 and the following
variables were found: hCMJ (R=- 0.68;p<SEE=0.92), h30CMJ (R=-0.75; p<0.01;SEE=0.82) h0.15 (R=-0.64; p<0.01; SEE=0.95) and
h15.30 (R=-0.76; p<0.01; SEE=0.81).

Discussion

The mean values we have observed on the CMJ
(54.95cm and 46.39cm, respectively for the sub49 and up49) are consistent with
the literature (Rusko et aI., 1993; Numella et aI., 1996; Badillo and
Gorostiaga, 1997). In the 30CMJ, our mean values (41.84cm and 37.15cm,
respectively for the sub49 and up49) are also consistent with a previous study
(Miguel, 2003), although information regarding this data is
scanty. The sub49 presented a better performance,
comparatively than the up49, as shown by a significant mean difference of 4.41 %
in the t400. Significant differences between the two groups were also observed
for all measures of the height of the rise of the cm (hCMJ, h30CMJ, h0.15 and h
15.30), which correspond to explosive strength (Es) and to speed strength
endurance (SSE) components (see Table 2). The sub49 presented higher mean values
and the differences between groups in those variables represented was between
10.06% and 15.58%. These results may suggest that to obtain significant
improvements in performance, the development of Es and the SEE should be more
than 10%. However, additional data from longitudinal studies with 400m runners
is warranted to support this suggestion. Moreover, the differences between
groups were larger on the second half of the 30CMJ, which emphasises the need to
develop SSE to attain top-level performance over 400m, as suggested by Harre Et:
Leopold (1987). We have found significant associations
between t400 and hCMJ (R=-0.683; p and between the t400 and h30CMJ (R=- 0.753; p
were pooled together (see Figure 1). In a previous study (Miguel, 2003), we have
also found a significant association between t400 and h30CMJ (R=-0.910;
p<0.005). When hCMJ and h30CMJ were included together to identify the
association with the t400, a significant association was found (R=-0.76; p
Therefore, we can hypothetise that those indicators of Es and SSE may be
sensitive to differences in the competitive level only when a large variation in
performance is present. Nevertheless, since the measurements that we have
performed were not necessarily coincident with each subject's peak performance,
additional data is required to confirm our hypothesis.

The indicators of Rs that were measured during the
15React test (PmaxRs and PmedRs) were not significantly different between groups
(see Table 2) and large differences were observed for these variables.
Additionally, there were no differences between the PmaxRs and the PmedRs, a
fact that suggests that a 15 jump test may be insufficient to indicate the
subjects' endurance ability in the Rs component. Moreover, those variables were
not able to discriminate the subjects from the two groups. Some authors set
emphasis on the importance of the reactive strength (or leg stiffness) to
maximal sprinting speed (Vittori, 1996; Chelly 8: Denis, 2001). Additionally, it
has been suggested that reactive strength (measured by 7 jumps) is associated
with maximal speed and speed maintenance in sprint events - 60m, 100m and 200m
(Locatelli, 1996; Lacour, 1996). It has also been suggested that performance in
400m events is dependent on the subjects' maximal sprinting speed (Mero et aI.,
1993; Rusko et aI., 1993; Numella et aI., 1996). However, we have not found any
association between the results in the 15React test and t400. Therefore, we may
conclude that Rs itself may not be a valid predictor of performance in the
400m.

Conclusion

The main finding of the present study was that
400m runners with different performance levels present different levels of Es
(as indicated by performance in the CMJ) and of SSE (as indicated by the
performance in the 30CMJ). Furthermore, we found that the Es and the SSE may
only be able to discriminate 400m runners when a large variability of
performance is present. Therefore, the development of these two strength
components seem to be an important issue to consider when designing preparation
programmes for 400m runners. Empirically, we know that in
their regular training programmes 400m runners usually include the following
contents for strength improvement (Miguel, 2002): jumps, weight training,
technique exercises and exercises to improve running specific-strength and
exercises for general conditioning. In events where the performance warrants a
combination of strength and endurance (events lasting less than one minute), the
training aims at the development of: i) endurance and aerobic strength
endurance; ii) maximal strength and explosive strength; iii) explosive strength
at high speed; iv) strength endurance and specific strength (Badillo and Serna,
2002). To Bosco (2000) the distinct strength components mentioned above are
commonly developed in sequence and in successive cycles in which a stage or
block evolution are expected. The author suggests that the SSE should be
developed in order that the maximal strength, the explosive strength and the SSE
should take place in a concurrent way. Whatever the
training programme model chosen by their coaches, we suggest that 400m runners
should take special care to perform exercises that aim to develop Es and SSE.
Thus, according to the suggestions of Bosco (1994, 2000) and Vittori (1991),
some of the exercises that can be used in order to develop Es are as
follows:

Concerning SSE development, it seems there is some
controversy regarding which methodology to use. However, we find the proposal of
Cometti (2001) interesting. The author suggests that the exercises should
include some general strength followed by the competition movement in a way that
the total exercise duration replicates the competitive event. For example: a
400m runner could perform a typical weight training movement (half squat, 10 RM
maximum) followed by an 80 to 100m sprint and repeat the sequence 3 times, which
means 50 to 60 seconds total duration of the exercise. Donati (1996) also
supports this type of combined exercises when the aim is to associate the
strength work with the specific speed work (these "associated exercises" can be
modulated in major or minor preponderance of strength or speed according to the
aims and needs and the possibility to develop several strength components in an
exercise sequence). Despite these interesting proposals
for the Es and the SSE training, our opinion is that further research is
warranted to confirm the suggestions, particularly concerning the SSE
development.

AcknowledgementsThis study was performed with the
support of the Laboratory of Investigation in Sport of the Sport Sciences School
of Rio Maior. We are grateful for the total availability of the athletes and
coaches to participate in this study, with special mention to the Sprints &
Hurdles Department of the Portuguese Federation of Athletics led by Dr. Jose
Carvalho. We also thank Dr. Felix Romero for helping in data handling and
statistical procedures.